Subject: Re: [PATCH v2 01/14] smp: Create a new function to shutdown nonboot cpus

From: Russell King - ARM Linux admin <[email-masked]>
Date: Tue, 21 Jan 2020 17:03:50 +0000

On Mon, Nov 25, 2019 at 11:27:41AM +0000, Qais Yousef wrote:
> This function will be used later in machine_shutdown() for some archs.
> 
> Signed-off-by: Qais Yousef <[email-masked]>
> CC: Thomas Gleixner <[email-masked]>
> CC: Josh Poimboeuf <[email-masked]>
> CC: "Peter Zijlstra (Intel)" <[email-masked]>
> CC: Jiri Kosina <[email-masked]>
> CC: Nicholas Piggin <[email-masked]>
> CC: Daniel Lezcano <[email-masked]>
> CC: Ingo Molnar <[email-masked]>
> CC: Eiichi Tsukata <[email-masked]>
> CC: Zhenzhong Duan <[email-masked]>
> CC: Nadav Amit <[email-masked]>
> CC: Greg Kroah-Hartman <[email-masked]>
> CC: "Rafael J. Wysocki" <[email-masked]>
> CC: Tony Luck <[email-masked]>
> CC: Fenghua Yu <[email-masked]>
> CC: Russell King <[email-masked]>
> CC: Catalin Marinas <[email-masked]>
> CC: Will Deacon <[email-masked]>
> CC: [email-masked]
> CC: [email-masked]
> CC: [email-masked]
> ---
>  include/linux/cpu.h |  2 ++
>  kernel/cpu.c        | 17 +++++++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index bc6c879bd110..8229932fb053 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -118,6 +118,7 @@ extern void cpu_hotplug_disable(void);
>  extern void cpu_hotplug_enable(void);
>  void clear_tasks_mm_cpumask(int cpu);
>  int cpu_down(unsigned int cpu);
> +extern void smp_shutdown_nonboot_cpus(unsigned int primary_cpu);
>  
>  #else /* CONFIG_HOTPLUG_CPU */
>  
> @@ -129,6 +130,7 @@ static inline int  cpus_read_trylock(void) { return true; }
>  static inline void lockdep_assert_cpus_held(void) { }
>  static inline void cpu_hotplug_disable(void) { }
>  static inline void cpu_hotplug_enable(void) { }
> +static inline void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) { }
>  #endif	/* !CONFIG_HOTPLUG_CPU */
>  
>  /* Wrappers which go away once all code is converted */
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index e2cad3ee2ead..94055a0d989e 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1058,6 +1058,23 @@ int cpu_down(unsigned int cpu)
>  }
>  EXPORT_SYMBOL(cpu_down);
>  
> +void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
> +{
> +	unsigned int cpu;
> +
> +	if (!cpu_online(primary_cpu)) {
> +		pr_info("Attempting to shutdodwn nonboot cpus while boot cpu is offline!\n");
> +		cpu_online(primary_cpu);
> +	}
> +
> +	for_each_present_cpu(cpu) {
> +		if (cpu == primary_cpu)
> +			continue;
> +		if (cpu_online(cpu))
> +			cpu_down(cpu);
> +	}

How does this avoid racing with userspace attempting to restart CPUs
that have already been taken down by this function?

-- 
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